OBJECTIVES: To conduct a systematic review of the literature on the safety of vaccines recommended for routine immunization of children, adolescents, and adults in the United States as of 2011. DATA SOURCES: We included placebo-controlled clinical trials and cohort studies comparing vaccinated and unvaccinated patients. We also included the following types of post-licensure analyses: case-control studies, self-controlled case series, and multivariate risk factor analyses. We conducted an electronic search of PubMed from inception through August 2013, and reviewed Advisory Committee for Immunization Practices statements, vaccine package inserts, and previously published reviews to identify studies. Scientific Information Packets were requested from vaccine manufacturers. REVIEW METHODS: We reviewed the methodology of the 2011 Institute of Medicine (IOM) consensus report "Adverse Effects of Vaccines: Evidence and Causality" and accepted their findings. We augmented their work with new studies and additional vaccines. For studies not included in the IOM report, we abstracted data on the presence or absence of adverse health outcomes, characteristics of patients, study design, and vaccine description, including brand, potency, dosage, timing, and formulation, where available. We excluded formulations not used in the United States. The McHarm instrument was used to evaluate the quality of adverse events collection and reporting in each study. We were unable to pool results; we rated the overall strength of evidence (SOE) as high, moderate, low, or insufficient by using guidance suggested by the Agency for Healthcare Research and Quality for its Effective Health Care Program. RESULTS: A total of 20,478 titles were identified; after title, abstract, and full-text review, 166 studies were accepted for abstraction. The vast majority of studies either did not investigate or could not identify risk factors for adverse events (AEs) associated with vaccination. Similarly, the severity of AEs was inconsistently reported, as was information that would make independent severity determination possible. SOE was high for the following associations in nonpregnant adults: seasonal influenza vaccine and arthralgia, myalgia, malaise, fever, pain at injection site; 2009 monovalent H1N1 vaccine and Guillain-Barré syndrome (GBS); and a lack of association between influenza and pneumococcal vaccines and cardiovascular events in the elderly. Risk of GBS was estimated at 1.6 excess cases per million persons vaccinated. SOE was high for the following associations in children and adolescents: measles, mumps, rubella (MMR) vaccine and febrile seizures in children under age 5; lack of association between MMR vaccine and autism spectrum disorders; and varicella vaccine and disseminated Oka strain varicella zoster virus with associated complications (i.e., meningitis, encephalitis) in individuals with demonstrated immunodeficiencies. There is moderate SOE that vaccines against rotavirus are associated with intussusception in children; risk was estimated as 1 to 5 cases per 100,000 vaccine doses, depending on brand. Moderate-strength evidence exists regarding human papillomavirus vaccine and a lack of association with onset of juvenile rheumatoid arthritis, type 1 diabetes, and GBS. Moderate-strength evidence shows no association between inactivated influenza vaccine and serious AEs in pregnant women. Evidence was insufficient to make conclusions regarding whether several routinely recommended vaccines are associated with serious conditions such as multiple sclerosis, transverse myelitis, and acute disseminated encephalomyelitis. CONCLUSIONS: There is evidence that some vaccines are associated with serious adverse events; however, these events are extremely rare and must be weighed against the protective benefits that vaccines provide. Careful consideration should be given to the investigation of research gaps, including patient risk factors that may be associated with AEs; however, important factors must be taken into account when determining whether studies are warranted, including the severity and frequency of the AE being studied and the challenges of conducting sufficiently powered studies when investigating rare events.